1. Technical Field
The disclosure relates generally to the field of marine geophysical surveying. More particularly, the disclosure relates to systems and methods for controlling the spatial distribution or orientation of geophysical sensor streamer or an array of geophysical sensor streamers towed behind a survey vessel.
2. Description of the Related Art
Marine geophysical surveying systems such as seismic acquisition systems and electromagnetic survey systems are used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or the ocean. Marine seismic surveying systems, for example, typically include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel is typically configured to tow one or more (typically a plurality) laterally spaced sensor streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy are ultimately conducted to the recording equipment. A record is made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may also be used in marine electromagnetic geophysical survey systems.
The one or more sensor streamers are in the most general sense long cables that have geophysical sensors disposed at spaced-apart positions along the length of the cables. A typical streamer may extend behind the geophysical survey vessel for several kilometers. Because of the great length of the typical streamer, the streamer may not travel entirely in a straight line behind the survey vessel at every point along its length due to interaction of the streamer with the water, among other factors.
Streamers towed by a vessel configured for towing multiple streamers are generally associated with equipment that maintains the forward ends of the streamers at selected lateral distances from each other and from the centerline of the survey vessel as they are towed through the water. Single streamers are generally used in what are known as two-dimensional geophysical surveys, and multiple streamer systems are used in what are known as three-dimensional and four-dimensional surveys. A four-dimensional seismic survey is a three dimensional survey over a particular area of the Earth's subsurface repeated at selected times. The individual streamers in such systems are generally affected by the same forces that affect a single streamer.
The quality of geophysical images of the Earth's subsurface produced from three-dimensional surveys is affected by how well the positions of the individual sensors on the streamers are controlled. The quality of images generated from the detected signals also depends to an extent on the relative positions of the sensors being maintained throughout the geophysical survey.
Various embodiments of streamer control systems and methods are disclosed in U.S. Patent Publication 2012/0002502, entitled “METHODS FOR GATHERING MARINE GEOPHYSICAL DATA,” which is incorporated by reference herein.